Aging is a physiological process of physical changes over time and is encountered by all living organisms. The human body is not immune to this process and is manifested in many forms. Just as hair begins to turn gray and skin begins to wrinkle, one sign of aging in the human body is the degradation of eye functions. In particular, all humans living beyond the age of their mid-40's begin to experience the effects of presbyopia. Presbyopia is the condition wherein the human eye exhibits a progressively diminished ability to focus on near objects and is most commonly experienced when reading.
The exact mechanisms of presbyopia are not known with certainty, however research evidence most strongly supports a loss of elasticity of the crystalline lens. The periphery of the crystalline lens is attached via the zonular apparatus of Zinn to the ciliary body which comprises the ciliary processes and the ciliary muscle and is located in the anterior portion of the human eye. One function of the ciliary body is to produce and regulate the flow of aqueous humor with respect to the anterior chamber of the eye. Another function of the ciliary body involves its musculature. The ciliary muscle is a ring of smooth muscle comprising longitudinal and circular fibers within the ciliary body in the eye's middle layer that, among its other functions, controls accommodation or changing of the shape of the lens within the eye for transitioning the focus of the eye between near objects and far objects. This dynamic mechanism of natural accommodation manifests in real time via a neural feedback loop involving the central and autonomic nervous systems wherein image quality on the retina drives ciliary muscle tone effecting the crystalline lens position and shape to achieve image clarity at varying object distances. The loss of accommodation continues such that by a person's 50's, intermediate vision is affected making it more difficult to work on a computer or read sheet music. By the time a person reaches 70 years of age, there is minimal to no accommodative capacity remaining in the eyes.
In a normally functioning eye, the cornea and crystalline lens of the eye work in concert to focus a clear image on the retina. The cornea provides the bulk of the focusing power and the lens provides the fine-tune focusing. The natural crystalline lens in the eye is flexible and according to the Hemholtz theory of accommodation the lens changes shape and becomes more globular upon ciliary muscle contraction allowing the focusing system of the eye to accommodate, or increase its dioptric power, providing focus at near distances. The degree of accommodation in humans declines with age such that at around 8 years of age, the range of accommodation is about 14 diopters. By the time a person reaches 40 years, the range has decreased to 6 diopters, and when a person reaches 68 years the range is further reduced to 0.5 diopters. Measurements in non-human primates indicate that the ciliary body still functions in mature adults and that presbyopia is ostensibly the result of increased stiffening of the natural crystalline lens.
Early non-surgical solutions to correct presbyopia include reading glasses or bifocal/progressive glasses and these presently remain the most commonly implemented remedial solutions for presbyopia. Contact lenses are now also being used to counter the effects of presbyopia through the use of “bifocal” (i.e., multifocal) contact lenses, or monovision where one eye is corrected for near vision and the other eye is corrected for distance vision. These are adequate solutions but do not correct the underlying problem and carry the inconvenience of dependence on glasses or contact lenses. While these solutions have disadvantages, they also have the advantages that they are not permanent and can be easily reversed and do not carry the attendant risks of surgical procedures.
Surgical options include such procedures as scleral relaxing incisions, laser thermokeratoplasty (LTK) and conductive keratoplasty (CK), corneal inlays, or accommodating/pseudo-accommodating intraocular lens implants. Scleral relaxing incisions have been found to be a poor solution at best. LTK/CK and corneal inlays treat the cornea and the effects of LTK/CK are generally not long-lasting, nor do these methods produce true accommodation, but are a version of either monovision or increased depth of field.
More recently, surgically implantable lenses have been developed to replace the eye's natural crystalline lens with an artificial lens in attempts to return accommodation back to the eye. The Crystalens®, an accommodating intraocular lens implant, is the first version of a monofocal lens designed to move in the eye in response to the natural movement of the ciliary body in accommodation. However, this lens does not provide sufficient dioptric range for good vision for near tasks. Pseudo-accommodating multi-focal intraocular lens implants (ReStor®, Array, ReZoom®, Tecnis MF®) provide adequate near vision but create optical distortions leading to less than ideal vision and do not provide the dynamic range of vision produced by natural accommodation.
Current intraocular surgical solutions for the treatment of presbyopia involve replacement of the malfunctioning crystalline lens. Multifocal lens implants are effective but compromise the quality of vision. Recipients of these lenses often experience visual artifacts including glare, loss of contrast sensitivity, and haloes. Further, these lenses are limited in the range of focus and do not provide a continuous dynamic range of focus from distance to intermediate to near distances. Currently, the only FDA approved accommodating intraocular lens (IOL) is the Crystalens® and has the advantage of being monofocal thereby providing a high quality image with minimal artifacts and distortion. However, the lens also has a downside wherein only a small range of accommodation is provided and the surgical results are variable, often requiring postoperative excimer laser surgery of the cornea for an ideal outcome. The lens only provides 1.5-2.0 diopters of accommodation whereas 2.5-3.5 diopters is needed for near vision tasks such as reading. An ideal IOL would have the advantages of a monofocal lens producing high image quality while minimizing glare and the other visual distortions of a multifocal lens. This ideal IOL would also provide a wide dynamic range of continuous accommodation of at least 4 diopters and couple seamlessly in real time to the natural neural and mechanical feedback mechanisms that are used to achieve accommodation in the human eye.